1. Field of the Invention
This invention relates to charged particle beam projection systems and more particularly to a method of design and manufacture of Charged Particle Beam Projection Systems (CPBPS).
2. Description of Related Art
For the purpose of lithography (among other things) in semiconductor electronics fabrication, a Charged Particle Beam Projection System as described in U.S. Pat. No. 5,466,904 of Pfeiffer et al. for xe2x80x9cElectron Beam Lithography Systemxe2x80x9d and U.S. Pat. No. 5,545,902 of Pfeiffer et al. for xe2x80x9cElectron Beam Lithography Systemxe2x80x9d employs Large Area Reduction Projection Optics with beam Scanning (LARPOS). LARPOS optical systems are based on providing a doublet of lenses for imaging a large object (integrated circuit pattern in a reticle), in combination with deflectors for positioning the image on the target (wafer) within a given range (scan field). There are numerous conceivable different combinations for providing such imaging/deflection, each characterized by the operating (input) requirements and performance in terms of image fidelity and positioning or exposure speed.
In turn, image fidelity is defined by the edge acuity, with which the pattern features are delineated in the exposure sensitive material (e.g. electron beam sensitive resist) on the wafer, as well as the trueness, with which the feature shape is reproduced. The former is often referred to in terms of the negative aspect of image xe2x80x9cblurxe2x80x9d. The result of lack of trueness of shape is often referred to as image xe2x80x9cdistortionxe2x80x9d. Both of these performance criteria are determined by the (charged particle) optical aberrations of the system, as well as Coulomb interactions between the charged particles (see below).
Among all CPBPS systems, LARPOS systems are subject to the largest number of aberrations or deviations of the individual particle trajectories from xe2x80x98idealxe2x80x99 or Gaussian optics. A class of aberrations is related to the lens system. Another class of aberrations is related to the deflection system. A third class of aberrations, so-called xe2x80x98hybridxe2x80x99 terms, is related to both.
The main goal for the designer of such a LARPOS system is to minimize the overall impact of the aberrations, under the specific condition of maximizing the beam current, as beam current (among other factors) is the primary factor determining the exposure speed. The consequence is that beam current by affecting the exposure speed ultimately affects both the throughput and the practical viability of a Charged Particle Beam Projection Systems (CPBPS) for industrial applications.
However, a problem with such a LARPOS system is that as the beam current increases the image blur and/or distortion, which become more pronounced. This is due to the effects of forces of electrostatic repulsion between charged beam particles, commonly referred to as Coulomb interactions. These Coulomb interaction effects are strongly dependent on several configuration and operating parameters, in particular, on the path length of the particles, and become worse as the path length increases. Thus, a major incentive for the design of such a system is to minimize the object-to-image distance of the system, i.e to design a tool in which the object-to-image distance is as short as possible.
This predicates overlapping of lens and deflection fields almost completely, giving rise to the need to optimize what is now a Curvi-Linear Variable Axis (CVA). The problem is to determine the optimum CVA in terms of blur and distortion.
In accordance with this invention, a method is provided for manufacturing a structure with an optimal CVA design by determination of the criteria of and for selecting the optimum CVA in terms of blur and distortion from a virtually infinite number of possible methods.
Further in accordance with this invention, a method is provided for making an optimized charged particle beam projection system by the following steps. Specify lens configuration and first order optics and then calculate lens excitations. Then configure the lens system thereby providing lens field distributions,and the beam landing angle and axis cross-over of the principal off-axis imaging ray. Then provide an input of a deflector configuration including an axial location of the deflectors, then solve linear equation set, and thereby provide a curvilinear variable axis and associated deflection field distributions, then calculate the third order aberration coefficients yielding a list of a plurality of (up to 54) aberration coefficients. Then provide an input of dynamic correctors. Next, calculate excitations to eliminate quadratic aberrations in deflection, then calculate third and fifth order aberrations, providing image blur and distortion vs. deflection, best focal plane, and depth of focus and calculate the total current consumption of all deflectors. Then test to determine whether the current result is better than the previous result, if YES then change the input for the axial location of the deflectors to solve the linear equation set again, if NO, then proceed to test whether the current result is acceptable, if NO, then provide a new deflector configuration input to again solve the linear equation set and continue through the steps thereafter, if YES, check as to whether the current consumption by all of the deflectors is higher than that of the preceding configuration. If YES, then change the input again. If NO then END the process.
Preferably when providing an input of a deflector configuration including the step of providing a beam trajectory, the radial component of which decreases monotonically from a reticle to an aperture placed at the axis cross-over location and increases from the aperture to a target.
Preferably, the lens system comprises an antisymmetric doublet which is preferably telecentric.
xe2x80x9cDoubletxe2x80x9d
The term xe2x80x9cdoubletxe2x80x9d as used herein denotes a pair of lenses operated under a specific symmetry condition, established in the following way:
A source (of particles) illuminates an object in front of a lens pair. The object is located precisely in the back focal plane of the first lens. The first lens generates an image of the source between the pair, and an image of the (closer) object at infinity. This effectively collimates the rays of particles emerging from the object. Accordingly the first lens is labeled xe2x80x9ccollimatorxe2x80x9d. The second lens is positioned exactly such that its back focal plane coincides with the front focal plane of the first lens. The second lens focuses the collimated, therefore parallel rays at its front focal plane, which then becomes the image plane for the object. Since the object is now projected into the image plane, the second lens is referred to as xe2x80x9cprojectorxe2x80x9d. Under this condition the optical magnification of the lens pair is given by the ratio of the focal lengths of projector to collimator, M=f2/f1. Simultaneously, the objectxe2x80x94image distance, is given by L=2(f1+f2). If lenses of the same shape are used, then their sizes scale with their respective focal lengths. For example, if f1=4f2, the collimator must be four times as large as the projector to maintain congruency of the lenses. Consequently, the plane of coincidence of focal planes between the lenses, located at a distance along the system (Z) axis from the object of z1=2f1, and by z2=2f2 from the image, constitutes a plane of symmetry. The doublet is xe2x80x9csymmetricxe2x80x9d about the coincidence plane. In the special case of f1=f2 or unity magnification, the doublet is xe2x80x9cmirror-symmetricxe2x80x9d.
If the source is placed infinitely far upstream of the doublet, its image will appear at the coincidence or symmetry plane. As a consequence, all rays originating from any one point on the source or its intermediate image at the symmetry plane will be parallelized by the projector. In that case, the doublet is characterized as a xe2x80x9ctelecentric symmetric doubletxe2x80x9d. If the lenses are of the magnetic type, their field polarities are generally chosen to be opposite to each other as to completely cancel the image rotation caused by each individual lens. One then speaks of an xe2x80x9cantisymmetric doubletxe2x80x9d.
The reason for operating the lenses in the described way as a doublet is that several aberrations are eliminated (as one lens compensates the aberrations of the other lens in exactly the right ratio) and consequently image blur is reduced.
Following the nomenclature of H. C. Chu and E. Munro (Optik 61(2) (1982) 121, pages 124 and 129), the first order optics of a lens system is characterized by two so-called fundamental rays or trajectories. One ray, denoted wa emerges from the object on the system/symmetry/central axis Z with unity slope, and the other ray, denoted wb, emerges from the object, (e.g. object R in FIG. 1,) at a point at unity lateral distance off the axis with zero slope, i.e. parallel to the axis. In the case of magnetic lenses, both have a radial and an azimuthal component, the former denoted ra and rb, respectively. The radial components lie, by definition, in a meridional plane, (i.e. a plane through the central axis Z as shown in FIG. 1).
The introduction of deflection devices into such a lens system has the purposexe2x80x94expressed in simple terms for illustrationxe2x80x94primarily of laterally shifting the optic axes of the lenses, such that the previously off-axis fundamental ray rb is now coincidental with the shifted lens optic axis. In that case it will no longer follow the original path, but will go straight through one of the lenses, and through the following lenses possibly off their optic axes, which may have been shifted by a different amount and/or in a different direction. In order to keep the fundamental ray on the optic axes of all lenses, additional deflection devices are needed, essentially to reconnect the individual axes, lens optic axes which have been shifted in various ways.
If the axis shifting operation is dynamic, i.e. variable in time, the system is called Variable Axis Lens or VAL system. Since the overall optic axis, shifted and reconnected for each lens by deflectors, is in general curved, it is called Curvilinear Variable Axis (CVA) and the system a CVA Lens (CVAL) system.